While it's not a big deal: A common implementation strategy in language run-times is to use machine words when the integer value fits within a particular range. This may mean that the shown implementation will incur a performance penalty well before it's actually necessary to pay the price for using big ints.
But this has nothing do do with binary search or mergesort.
Regardless of our opinions of overflow, it exists in many languages. And I cannot think of anything that would better qualify this issue for "has something to do with binary search" status than that most programmers have seen, used, or written an implementation with this bug.
If Perlis was right when he said that Lispers know the value of everything and the cost of nothing, this bug proves the counterpoint: of Fortran-derived languages knowing the cost of everything but the value of, um, most things but not some others.
But this has nothing do do with binary search or mergesort.
Regardless of our opinions of overflow, it exists in many languages. And I cannot think of anything that would better qualify this issue for "has something to do with binary search" status than that most programmers have seen, used, or written an implementation with this bug.